To build, you must mix some concrete (5)
I believe the answer is:
erect
'to build' is the definition.
(I know that erect is a kind of build)
'mix some concrete' is the wordplay.
'mix' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'some' indicates the central letters.
'concrete' becomes 'objective' (similar in meaning).
The central letters of 'objective' are 'ect'.
'er'+'ect'='ERECT'
'you must' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for erect that I've seen before include "Build, upright" , "Build, put up" , "Rigidly upright" , "Upright; construct" , "Upright; put up" .)